[berkmberk1]

Considering the law specifically dictates that it is legal (mailing a cartridge gun to the factory or other authorized entity, or a bp to any authorized entity - given local restrictions), I see no reason not to package it up very well, wrap it up in brown paper, take it to the post office, insure it for what its worth, and don't tell them what it is. You don't HAVE to tell them. There are no declarations you have to make, like flying on commercial aircraft. If, by some unforseen circumstance, a postal employee were to open the package, there would be nothing they could do. A really dumb one would notify the postal police/inspectors (yep, they exist) and I bet it wouldn't take two seconds for that employee to be set right.

If you don't break the law, a dumb government employee can't get you in trouble.